Digging with high water table

[Tony L]

@Alan Ambrose you asked me to report back when we started to dig the trench foundations.

 

Just to recap, we have a high water table.  I did some research - it's usually at its lowest during the first week in October, so I booked the groundworks team.  Then we had the wettest September I can remember.  We dug some test holes a while back & the BCO came for a look & told us we'd need 1M deep trenches.  This is what had been specified on the new build right next door to us.  We were expecting to have to shutter the trenches to prevent them collapsing between the dig & the pour. A different BCO came to inspect the trenches once we'd started digging.  He told us he wanted to see 2M.  Due to the ground conditions, this work has been very much more difficult than digging 1M trenches.  As the pictures show, I don't have a nice, neat 60cm wide trench describing the outline of my house.  The reclaimed boards I'd collected to deal with the shuttering were deemed too weak to risk using for these ground conditions.  OSB (as they used next door) would also be too weak, so the pictures show lots of very expensive plywood, & 2 x 4 braces (these won't be left in).  The water you see in the pictures is being pumped - if the pump is off overnight, it's up over the boards the next day.  

[Tony L]

[ProDave]

We have a seasonal high water table, Mid summer perhaps June or July is the driest.  I would never have thought of digging in September let alone October.

 

We were about 1.5 metre without shuttering but needed a pump to keep the water out.  BCO inspection and pour same day.

[saveasteading]

Why did the bco want 2m after the original spec was 1m?

Thanks fof the report. Is it going to be OK, or are there any queries?

[Alan Ambrose]

@Tony L Thanks for that - that does look a bit tricky - but congrats on handling the problem. How big a pump did you need? I've been talking to some basement guys (Keller) over the past few weeks, and they said they don't mind clay as the inflow of water is normally fairly slow, unlike sand.

 

>>> I did some research - it's usually at its lowest during the first week in October

 

That's interesting - did you find some stats somewhere?

[Tony L]

4 hours ago, saveasteading said:

Why did the bco want 2m after the original spec was 1m?

Thanks fof the report. Is it going to be OK, or are there any queries?

The BCO who'd inspected our 2M test holes was away, so his colleague came along & he had a different view.  One option would have been to stop work & initiate a protracted argument with BC, but my builder decided to do as he was told, & I'm happy with his decision.  

 

Our next inspection is due tomorrow - LA BCO wants to check everything immediately before the concrete is poured on Friday.  I think it will be OK.  The warranty inspector doesn't want to come again until he can see DPC.

[ToughButterCup]

4 minutes ago, Tony L said:

The BCO who'd inspected our 2M test holes was away, so his colleague came along & he had a different view. ....

 

Grrrrrr.......

[saveasteading]

It doesn't seem that you need advice.

But just in case.

It's usually best to dig slightly short as it will get muddy and maybe soft and have to come out. So let bco tell you to take out 50mm more down to 2m. 

 

Plus it's OK to pour concrete into a wet trench but it must flow not splash. It will push the remaining water to the end and can be bailed out. Otherwise it mixes and a wet mix is a bad thing.

[Tony L]

3 hours ago, Alan Ambrose said:

@Tony L Thanks for that - that does look a bit tricky - but congrats on handling the problem. How big a pump did you need? I've been talking to some basement guys (Keller) over the past few weeks, and they said they don't mind clay as the inflow of water is normally fairly slow, unlike sand.

 

>>> I did some research - it's usually at its lowest during the first week in October

 

That's interesting - did you find some stats somewhere?

 

I don't know the spec' of the builder's pump that's being used.  It's small - it would fit in a regular sized builder's bucket.  I bought a similar (slightly smaller) dirty water pump from ebay for about £20, to keep my test holes dry when we dug them - it just sat in a bucket with a few holes drilled in the sides (so it wasn't in the mud).  If I'd been managing the dig myself, which was the original plan, I was going to look out for another couple of pumps the same, to be sure I had enough pumping power.  If you get them used from ebay you can likely sell them without loss, once the job's done.  If you buy a second hand pump, make sure it comes with the hose.

 

Yes, I found stat's & graphs with water table heights for my area.  Sorry, I'm too busy to try to find these again now.  I should have made an allowance for the recent climate change effect & got organised to start in August.  It looks like the warmer, wetter winters that climate change scientists were promising us for the UK 15 or so years ago are with us now.

[Tony L]

11 minutes ago, saveasteading said:

It doesn't seem that you need advice.

But just in case.

It's usually best to dig slightly short as it will get muddy and maybe soft and have to come out. So let bco tell you to take out 50mm more down to 2m. 

 

Plus it's OK to pour concrete into a wet trench but it must flow not splash. It will push the remaining water to the end and can be bailed out. Otherwise it mixes and a wet mix is a bad thing.

Thanks for this.  I need as much advice as I can get really - I'm a total novice.  Fortunately, I have full confidence in my builder & his team.  I wish I could say they same about the Arch Tech, who's turning my design into proper drawings.

 

I think they're going to pump the trenches as dry as possible before they pour so they can get all the battens out - some of these are below the level of where the concrete's going.  I didn't ask, but I guess the builder decided that on some sections, the pegs that are holding the bottom of the plywood in place aren't strong enough to last several days on their own.

[saveasteading]

2 hours ago, Tony L said:

have full confidence in my builder & his team.  

Great to hear. Who pays for the extra work and concrete?

 

Ask away.

[Tony L]

I pay, because the quote I accepted was based on 1M trenches & me supplying all the wood for the shuttering (which I did, but the builder said it was too risky to use my old reclaimed ply & OSB).  Fortunately, the spec' on the poured concrete hasn't changed - it's still 600 wide & 600 deep, but there's an extra metre of block work to go on top now, & a lot of extra muck away.  I'll report back once I have the bill for the additional work.

[Iceverge]

What a pain. 

 

It seems you are managing it well. 

 

Good luck with the pour.  

[Tony L]

Thanks for the encouragement.  I said I'd update, so:

 

Confidence in the builder is waning.

 

I'm too busy to trouble you with all the details right now, but the bill for the first lot of extra work was £17,700. The BCO made a second visit & decided a tree he hadn't seen on his first visit would require yet another plan to be devised.  This second BCO visit resulted in another bill for extras from the builder.  I said, "But now you're not building everything that was included in the first bill for extras. The invoice lines on the 2nd bill might be OK, but you should have gone on to deduct some of what was on the first bill".  The builder has now agreed a small deduction should have been made, but I think it should be much bigger.  His explanation of the various costs is not clear.  I want it all explained in an email so I can pick through it carefully. He says he's not good at email & he's pressurising me to run through it all at a site meeting.

[joe90]

1 hour ago, Tony L said:

He says he's not good at email

Get it in writing (of any sort).

[saveasteading]

2 hours ago, Tony L said:

The BCO made a second visit & decided a tree he hadn't seen on his first visit would require yet another plan

It isn't his job to do all the detail, including assessing tree type, ground type and the distance. He does spot checks. 
and you are the client and aren't expected to know any of it, so it is the builder or designer.

 

Just in case it helps still....the tree affects foundations according to distance. eg if it is 15m from the building and the found's have to be deeper, then it will be further  than that to any other areas of foundation, and they are shallower.

 

[Iceverge]

I looked at the first picture you posted and held my head in my hands.

 

In a different world you might be able to just lock the site gate and come back in 6 months,or a year, or two years when conditions were favourable. Alas impractical with all the paperwork following building .

 

Another option would be to park the shovels and design some easier to build foundations like piles with a ringbeam or such. 

 

However we are where we are etc.....

 

Before trying to pin your builder to the wall with emails maybe try to climb into his boots for a second. Take him to a nice cafe somewhere, on the clock, buy some buns, and have a grown up discussion. Importantly, listen to him. 

 

I know it's frustrating to see the budget and schedule slipping but I think it's the ground eating it. Not your builder. 

 

Resist for now the temptation to bolster the moral of men, up to their knees in sh*t, with dictatorial emails. 

 

Edited Monday at 23:46 by Iceverge

[lookseehear]

Plenty of sympathy here - we have been battling with went trenches too, albeit on a smaller scale (35m2 extension). Foundations that were going to be 500mm thick ended up 900mm in places due to having to keep taking the top layer of clay off as it got soft and soggy. Our OSB shuttering held up OK, but was a bit too close for comfort in places and wouldn't have been sufficient on a bigger project. Our biggest issue was that we can't get a digger into the bottom of the trenches, so a lot of horrible digging and excavating by hand.

 

The thing that helped in the end was having a pump sat in a small sump running on a timer rather than on the float so that the water didn't get above the top of the sump regularly. This helped us get it under control while finishing the digging, then got BCO approval the day before the pour. Groundworks are hard, depressing (literal) money pits, but once you're through this bit and are standing on clean concrete it will feel worth it.

 

Agree with Iceverge about keeping the builder on side. From his perspective he may feel that if he makes a mistake in an email then he may be held to it. Try if you can to talk it through somewhere that isn't on site and take your time. This part is stressful for all involved and if the builder is otherwise someone you trust will do a good job then it's worth trying to get through it.

 

Here's a picture of our trench prior to foundations in solidarity.

 

[Tony L]

On 18/11/2024 at 23:39, Iceverge said:

I know it's frustrating to see the budget and schedule slipping but I think it's the ground eating it. Not your builder. 

 

 

I've duplicated some of this story in my separate trench blocks thread.  Thanks for all the responses here.  The work continues.  I have plenty of sympathy for the builder.  He is working hard in conditions that are more difficult than we anticipated.  I agree, the ground is eating my budget, & the BCO has cost me a fortune by telling me to pay for something to be done then telling me, on his next visit, that what he asked me to do (& I've paid for) is not what needed to be done, & a new plan needs to be devised & paid for.  My email to the builder was not dictatorial; I set out some materials calculations & asked him to comment on them or correct them.  I was seeking clarity on how much of what I'd paid £17,000+ for had been done & how much had not been done - ie how much of the £17k+ should have been deducted from the subsequent invoice I received, which covered implementing the new plan that was required because the BCO didn't tell us he was concerned about the tree, on his first visit.  The new plan replaced the £17k plan - it wasn't a case of, "Do everything I told you to do & do this extra work as well.".  

 

I'm more inclined to agree with Joe's view, "get it in writing", than pay for an off site meeting during which the builder must attempt to explain loads & loads of figures & a fair amount of maths.  There's too much to take in if I'm listening to it all, rather than steadily working through an email I've printed out & can write notes on.  We've had a good meeting since our email exchange. 

[Alan Ambrose]

>>> BCO has cost me a fortune by telling me to pay for something to be done then telling me, on his next visit, that what he asked me to do (& I've paid for) is not what needed to be done, & a new plan needs to be devised & paid for.

 

I appreciate that BC often seems to have some special laws of nature for itself … but … it’s not unreasonable, unless new information has arrived, to expect some consistency of thinking and thorough explanations for any changes of position. Not ‘that’s just what I’m thinking today, it might be different tomorrow’. Take this up politely with their management and ask for a 2nd opinion?

[Tony L]

I've left lots of detail out of my story, because there's a lot to it & a full explanation would be so lengthy nobody would want to read it.  The 1m instructions were given when a BCO visited the site & inspected 2m holes I'd dug either side of the house location around 3 years ago, before we properly got going with the build.

 

I didn't think we had time for an argument.  The ground was unstable & the trenches could have collapsed.  The shuttering had been engineered to last just a few days & the builder & I thought, if we don't do whatever needs to be done quickly, we could end up having to spend even more money to satisfy BCO.

 

I was discussing this with another local builder a couple of days ago & he explained BC are a lot more strict on foundations than they used to be just a few years ago, especially when it comers to nearby trees.  

[Macfracam]

New members here - can I tap into the expertise on this thread or should I start another? We are also facing high water table issues. We are finalising plans for a bungalow and are looking at options for the slab. Bedrock is chalk with sand and gravel on top. A soakaway test today revealed that we have 600mm soil and then 300mm gravel/sand before we hit the water table (which rises more if it is a wet winter). We are hoping to use an insulated pad, which would sit on crushed stone on top of the gravel. Does that seem a feasible option?

[saveasteading]

12 hours ago, Macfracam said:

Bedrock is chalk with sand and gravel on top. 

A new thread will be best I think.

Chalk is my second favourite bed material so it's a good start.

It works as rock for foundation strength, digs neatly and safely and drains reasonably. Turns to toothpaste when driven on, is the downside.

 

With the chalk there, my instinct is for conventional footings. 

600mm soil. That isn't topsoil presumably. What is it?

[Macfracam]

Thanks. Looking at the measurements again, it is 300mm topsoil, 300mm sandy soil with some gravel and then sand/gravel/chalk mix. The solid chalk is mostly below the water table we think although we haven’t done a lot of investigation. We will ask the SE about traditional options. The insulated slab we are looking at is extruded PS so is meant to be completely waterproof, helping if the water table rises in the winter.

[saveasteading]

10 minutes ago, Macfracam said:

extruded PS so is meant to be completely waterproof, helping if the water table rises in the winter.

A sheet of damp proof membrane under the concrete slab does the same for 50p/m2.  

 

The ground sounds great. The 300mm topsoil has to go, and be replaced by stone.

If you can stick to traditional construction methods, the cost is much lower, local builders can do it, and to some extent it is more adaptable to any particular details you want.

 

Why are you worried about high water? does it flood/ puddle visibly?   

Another thread is preferable as we are hijacking the original.

@Admin can apparently shift it.